class Solution {
    public String toLowerCase(String s) {
        StringBuilder ret = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (ch >= 'A' && ch <= 'Z') {
                char tmp = (char) (ch + 32);

                ret.append(tmp);//拼接

            } else {
                ret.append(ch);
            }
        }
        return ret.toString();
    }

    public static void main(String[] args) {
        String  str=new String("wyb");
    }
}
